Radioactivity bore hole logging



May l0, 1960 A. s. MCKAY 2,936,375

RADIOACTIVITY BORE HOLE LOGGING Filed DGO. 27, 1955 Tlc' .1. L 40 ,ber

gamma rays from the source.

The cable at the Ysurface passes over a suitable measuring reel or drum22 which, in any well-known manner, provides an indication or record ofthe amount of cable payed out and thus the depth of theinstrument 18 inthe bore hole at any time.

Shown as within and near the bottom of the instrument housing 18 is asource 24 of primary radiation suchv as neutrons and also within thehousing and above the source is a detector 26 of secondary radiationsuclr as gamma rays which may be induced in the surrounding formationsdue to bombardment thereby by the neutrons from the source 24. Mountedbetween the source 24 and detector 26 is a direct radiation shield 28forpreventing primary radiation from the source from passing upwardly tostrike the detector 26. As is well known such a shield may comprise oneor more layers of a suitable radiation absorbing substance, e.g., leador tungsten in the case of gamma rays and paran and cadmium in the caseof neutrons.

In the case where the source 24 emits both neutrons and gamma rays theshield 28 may `comprise a lower layer of paraflin or some othersubstance for slowing down the neutrons from the source, an intermediatelayer of cadmium for absorbing the slowed neutrons and an upper ,layerof lead or tungsten for absorbing The output from the detector 26 isshown as passing to a suitable preamplier and power supply 30 which isconnected in turn to the conductor cable 20. At the surface the outputof the preamplier 3i) is conducted from the cable to an amplier 32, theoutput of which passes to a galvanometer or the like, shownschematically at 34. Extending from the galvanometer 34 is a pen arm 36adapted to engage a Vmoving record strip.38 of the recorder 40.

With the apparatus thus far described, when the instrument 18 is loweredor raised through the bore hole 10 neutrons from the source 24 passoutwardly to bombard the material surrounding the instrument, suchmaterial including the well uid in the hole and the `surroundingformations such as theone indicated at 12. By nuclear reaction with theatoms of thesurrounding materials gamma rays may be induced, some ofthese traveling in -a direction such as to strike the detector 26 in thebore hole. If the detector is of the electrical pulse producing, gasamplification or countertype, pulses are produced therein by the gammarays and these pulses are preamplied at 30, passed over the cable 20,again amplified and usually integrated in the device 32 to provide avarying direct current potential to the galvanometer 34. The pen arm 36records on strip 38 a curve 42 indicating the output of the detector 26.As has been pointed out hereinabove, the number of induced gamma raysstriking the detector 26 and thus the detector output will depend uponthe hydrogen content of the material surrounding the source and detectorand the detector output will, depending upon the spacing between thedetector and source,

increase or decrease with greater or lesser amounts of curve 42 wouldindicate only the hydrogen content of the formations and thus theirporosities, but a bore hole -with no fluid therein is seldom if everencountered.

Againif the walls of the hole were uniformly straight with no cavitiesor enlarged portions, the eifect of the hydrogen-containing uid aroundthe instrument would be uniform throughout the hole and could easily bedisregarded or compensated for. However, since there is almost alwaysfluid in thebore hole and since the walls of the hole are not uniformlystraight but usually contain enlargement such as is indicated at 14 thiseifect of the varying amounts of hydrogen-containing uid around theinstrument should not be disregarded or a poor or incorrect log willresult. In order to overcome this trouble the apparatus to be describedhas been provided.

Pivotally attached to the wall of the instrument housing 18 are one orpreferably three or more arms 44 each arm having at its outer end asmall feeler or wheel 46. The upper end of the arm 44 is shown asextending within the housing 18 and a small tension spring 46 attachedto the upper end of the arm and with its outer end axed to the housingserves to bias the arm so that its lower end will engage the formationwalls. The upper end of the arm 44 is shown as contacting an electricalresistance element 48, its position along the element depending upon theposition of the arm and thus the diameter of the bore hole. The resistor48 is shown as connected through the cable 20 to an amplifier 50 andalso through the connector 52 to another galvanometer 54 having a penarm 56. Movements of the arm 44 caused by variations in the size of thebore hole cause more or less of the resistance 48 to be connected incircuit with the galvanometer 54, and as the instrument 18 moves throughthe bore hole in correlation with the movement of the record strip 38the pen'arm 56 will produce a curve 58 on the strip which willconstitute a caliper log, i.e., a record of the variations in the borehole size. The caliper curve 58 is also shown on the enlarged section ofthe record strip 38 in Fig. 2 as well as the induced gamma ray curve orlog 42. A bore hole calipering apparatus somewhat similar to thatdescribed above is disclosed in the U.S. Letters Patent No. 2,369,672,granted February 20, 1945, D. G. C. Hare.

In order to facilitate the proper interpretation of the induced gammaray log a plurality of reference lines or curves covering the range ofearth porosities from 0% to 40%, which lines are related to the borehole diameter, are recorded on the record strip 38 along With theneutrongamma ray curve 42 and the caliper curve 58. The proper positionof the reference lines as a function of porosity or hydrogen content andbore hole diameter can be determined by passing the logging instrumentthrough a bore hole in which these characteristics are known. As 'onemeans of providing and recording the reference lines one form ofapparatus is illustrated schematically in Fig. 1 and is described below.

The movement of the caliper arm or arms 44 will provide at the surfacethrough the cable connection 60 a varying voltage which can then bemodified in any suitable mauner such as by a non-,linear electroniccircuit or by a suitable motor system controlling a plurality ofvariable resistors in such a way that the reference lines will appear onthe record strip in their proper positions. Thus the varying voltagefrom the caliper tool is passed to the amplifier 50 where it may beamplified, if desired, and passed over a connector 62 to a galvanometer64, the pen arm 66 of which will trace on the strip 38 a curve 68showing the Variation in counting rate or detector out- 'put withvariations in bore hole diameter for the case 'to rotate slightly in aclockwise or counter-clockwise direction depending upon the output ofthe amplifier 50 and thus the Variations in the size of the bore holeand the pinion 76 engages a toothed rack 78 which will movecorrespondingly along its longitudinal axis. Afiixed to the rack 78 area plurality of pointers or contact members 80 each member being adaptedto make contact with a respective electrical resistor 82. Each resistor82 is connected to a different galvanometer 84 and each galvanometer hasa pen arm -for producing a trace or curve such as 86 on the record strip38. The resistors 82 are preferably non-linear for several reasons. Forexample, with a small diameter bore hole and thus a low amount ofhydrogen in the surrounding fluid then the addition of a certain amountof hydrogen around the inassesses strument provides a muchlargerdiference in counting rate thanin the casewith a largerdiameterhole with a 'correspondingly greater amountY of thehydrogen-con- ,taining iluid around'thefinstrument. vThe resistors 82are selected sothat their characteristics will be in accordance with theresults of the above-mentioned tests of the instrument in a bore holewherethe -bore hole diameter and hydrogen content of the surroundingfluid has been ascertained. Thus the upper resistor 82 will have a valuesuch that lthe galvanometer 84 to which it is connected will provide areference line 88 on the Yrecord strip 38 to indicate porosity and theother resistors 82 will be ,correspondingly selected so that theirgalvanometers will provide reference lines indicating 30% and 40%porosity as shown in Fig. 2.

With reference to Fig. 2, the enlarged portion 14 of the bore hole isindicated by the hump 90 in the caliper log 58 andthe reference lines68, 88, etc. will indicate ytheir'respective porosities for the holehaving the enlargement 14. The neutron-gamma curve `42 is, in effect,superimposed on the reference lines and the porosity or hydrogen contentof the formations is measured at any depth by noting the position of thecurve 42 With respect toY the `reference lines. Thus, by interpolationat the depth indicated at, say 92, the porosity or hydrogen-contentWouldbe indicated as about Again, at the depth or point 94 where theneutron-gamma curve crosses the reference line indicating 20% porosity,the porosity would be shown to be exactly 20%.

Although with the arrangement shown in the drawing the instrument 18would travel substantially alongthe longitudinal axis of the bore holesince the arms would center lthe instrument therein, insome instanceswhere a largev diameter hole is to be logged or where large wallcavities or enlargements are to be expected it may be better to maintainthe logging instrument close to or in contact with one side of the holewall. Such an arrangement is illustrated in Fig. 2'of the aforementionedHare Patent No. 2,369,672.

l. It will be noted that in Fig. 1 of the drawing the feeler 46 engagesthe bore hole wall at a point above the source 24 and detector 26. Againthis is for sake of clarity in illustration. It is to be understood thatfor practical reasons the feeler 46 should be mounted so that it willengage the formation walls at a point substantially midway betweenthesource 24 and the detector 26. However, the apparatus as illustratedin Fig. 1 can also be used by providing any suitable'means at thesurface for slightly retarding the recording of the caliper log 58 andthe curves 6,8, 88, etc. on the record strip as comparedv to theneutron-gamma ray curve 42 while the instrument is moving upwardlythrough the hole. VI'n such a manner, even when the caliperinginstrument is. not horizontally opposite the source and detector theseveral curves can bel positioned laterally opposite each other for easein interpretation as is the case in the showing of Fig. 2.

It is also to be understood that it is not essential that `all of thecurves or reference lines be recorded simultaneously. Thus, the logginginstrument could be passed through the hole two different times, theneutron-gamma ray log being made at one time and the caliper log andreference lines the other time, it merely being necessary that theneutron-gamma curve be superimposed on the reference lines, or viceversa, as indicated in Fig. 2.

While the invention has been described with relation to the making of aninduced gamma ray log, it is to be understood that the same principleswill apply Where certain other logs are made such as a scattered neutronlog or a scattered gamma ray log. In the iirst case, the detector 26would be a neutron detector and in the second case the source 24 wouldbe a source of gamma rays rather than neutrons.

Obviously, many modifications and variations of the invention ashereinbefore set forth may be made without v departing from the spin'tand scope thereof and therefore t 6 Y Y only such limitations should bevimposed as are indicated in the appended claims.

I claim: f

l. The method of making Ian improved induced gam# f ma ray log todetermine the hydrogen content of'earth formations traversed by a borehole which comprises passing through the hole a source of neutrons so asto [bombard said formations thereby and induce' gamma rays therein,detecting the intensity of said induced gamma rays in a zone in the hole.near said source, recording on a moving strip a curve of the intensityof the detected induced gamma rays in correlation With depth in thehole, simultaneously varying a voltage Iin accordance with variations inthe diameter of said bore hole opposite said zone, simultaneouslyrecording on said strip in correlation with depth in the hole aplurality of reference curves each corresponding to a different value ofsaid varying voltage rand thus to different values of the hydrogen con-Vtent in the material surrounding said source, in order to provide arecord with the induced gamma ray curve plotted in correlation withcorrected reference curves.

2. The method described in claim 1 with the additional steps ofrecording on said record strip a caliper curve produced by said Varyingvoltage, said curve .heingpositioned on said strip laterally oppositesaid induced gamma ray curve and said reference lines so that points onsaid curves and lines laterally opposite each other Ywill'represent thesame 'depth in the bore hole.

3. The method of making lan improved induced gamma ray log to determinethe hydrogen content of earth formations traversed by a bore hole whichcomprises bom- Y.

formations, detecting the intensity of said induced gamma rays in ameasuring zone in the hole near said source, recording on a movingrecord strip a curve of the iritensity of the detected induced gammarays in correlation with depth in the hole, simultaneously'varying avoltage in accordance with variations inthe diameter of said bore holeopposite said zone, simultaneously recording on said strip incorrelation with depth in the hole a plurality of reference curves eachcorresponding to .a different value of said varying voltage and'thus toYa different percentage of the hydrogen content of the bore hole uidsurrounding said detecting zone, in `order to provide a record with theinduced gamma r-ay curve plotted in correlation with corrected referencecurves.

4. The method described in claim 3 with the additional steps ofrecording on said record strip a caliper curve produced fby said varyingvoltage, said curve being posi-` tioned on said strip laterally oppositesaid induced gamma ray curve and said reference lines so that .points onsaid curves and lines laterally. opposite each other will represent thesame depth in the bore hole. t

5. The method of making -an improved radioactivitylog to determine Iacharacteristic of earth formations traversed by a bore hole whichcomprises passing through the hole a source of primary radiation so `asto lbombard said formations thereby and induce secondary radiationtherein, detecting the intensity of said secondary radiation in a zonein the hole near said source, recording on a moving strip a curve of theintensity of the detected secondary radiation in correlation with thebore hole depth, calipering the bore hole in substantially the samehorizontal plane as said detecting zone, simultaneouslyY in order toprovide a logging record with the detected radiation curve indicative ofsaid characteristic plotted with respect to reference lines that havebeen continuously corrected for the effect of variations in bore holediameter.

.6. The method of making an improved induced gamma ray log to determinethe hydrogen content of earth formations traversed by a bore hole whichcomprises passing through the hole a source of neutrons so as tobomrnard said formations thereby and induce gamma rays therein,detecting the intensity of said induced gamma rays in a zone in the holenear said source, recording on a moving strip a curve of the intensityof the detected induced gamma rays in correlation with the bore holedepth, simultaneously calipering the bore hole in substantially the samehorizontal plane as said detecting zone, simultaneously recording onsaid strip a plurality of reference lines, each of said reference linescorresponding to a different hydrogen content in the materialsurrounding said source, and controlling the value of said referencelines as a function of the caliper measurement, thereby providing a logindicative of said characteristic and which is recorded With respect toreference lines that `have been corrected for variations in bore holediameter.

7. The method of making an improved radioactivity well log to determinea characteristic of earth formations along the bore hole comprising thesteps of passing a radiation detector through the bore hole fordetecting radiation entering the bore hole from said earth formations,the intensity of the detected radiation being subject to variation las afunction of variation in the diameter of the bore hole fromplace-to-place along the Well bore, making a substantially continuousrecord of the intensity of the `detected radiation on a record strip incorrelation with the position of the detector in the bore hole,calipering the bore hole along the formations traversed by saiddetector, recording a plurality of reference lines on said record strip,each of said reference lines corresponding to a different percentage ofthe characteristic of the formation indicated by the recorded value ofradiation intensity, and continuously controlling the value of saidreference lines in correlation with the record of detected radiation asa function of said caliper measurement, in order to provide a loggingrecord with the detected radiation record indicative of saidcharacteristic recorded with respect to reference lines that have beencontinuously corrected for variations in bore hole diameter.

8. The method of making an improved Well log to determine acharacteristic of earth formations along the bore hole comprising thesteps of passing a device through the bore hole for determining aquantity indicative of said characteristic, the determined quantitybeing subject to variation as a function of variation in the diameter ofthe bore hole from place-to-place along the well bore, making asubstantially continuous record of the value of the determined quantityon a record strip in correlation with the position of the device in thebore hole, calipering the bore hole along the formations traversed byvsaid device, recording on said record strip a plurality of referencelines, each of said reference lines corresponding to a differentpercentage of the characteristic of the formation indicated by therecorded value, and continuously controlling the value of said referencelines as a function of said caliper measurement, in order to provide alogging record with the detected radiation record indicative of saidcharacteristic recorded with respect to reference lines that have beencontinuously corrected for variations in bore hole diameter.

9. The method of making an improved radioactivity well log to determinea characteristic of earth formations comprising the steps of traversingthe bore hole opposite said formations with a detector of radiationentering the bore hole from said earth formations, making asubstantially continuous record of the intensity of the detectedradiation on a record strip in correlation with the position of the`detector in the bore hole, during the same traversing step caliperingthe bore hole in the vicinity of said detector, simultaneously recordingon said strip reference data corresponding to a predetermined value ofthe characteristic of the formation indicated by the recorded radiationintensity, and controlling the value of said reference data as afunction of said caliper measurement, thereby providing a log indicativeof said characteristic and which is recorded with respect to referencedata that have been corrected for variations in bore hole diameter.

10. The method of making an improved well log to determine lacharacteristic of earth formations along the traverse of the bore hole,comprising the steps of traversing the *bore hole opposite theformations with a device for determining a quantity indicative of saidcharacteristic, the determined quantity being subject to variation 4as afunction of variation in the diameter of the bore hole fromplace-to-place along the well bore, making a substantially continuousrecord of the value of the determined quantity in correlation With theposition of the device in the bore hole, during the same traversing stepcalipering the bore hole in the vicinity of said detector,simultaneously recording on said record reference data corresponding toa predetermined value of the characteristic of the formation indicatedby the recorded value and controlling the value of said reference dataas a function of said caliper measurement, thereby providing a logindicative of said characteristic and which is recorded with respect toreference data that have been corrected for variations in bore holediameter.

References Cited in the le of this patent UNITED STATES PATENTS2,369,672 Hare Feb. 20, 1945 2,515,535 Thayer et al. July 18, 19502,648,012 Scherbatskoy Aug. 4, 1953 2,648,778 Silverman et al. Aug. 11,1953 2,761,977 McKay Sept. 4, 1956

1. THE METHOD OF MAKING AN IMPROVED INDUCED GAMMA RAY LOG TO DETERMINE THE HYDROGEN CONTENT OF EARTH FORMATIONS TRAVERSED BY A BORE HOLE WHICH COMPRISES PASSING THROUGH THE HOLE A SOURCE OF NEUTRONS SO AS TO BOMBARD SAID FORMATIONS THEREBY AND INDUCE GAMMA RAYS THERIN, DETECTING THE INTENSITY OF SAID INDUCED GAMMA RAYS IN A ZONE IN THE HOLE NEAR SAID SOURCE, RECORDING ON A MOVING STRIP A CURVE OF THE INTENSITY OF THE DETECTED INDUCED GAMMA RAYS IN CORRELATION WITH DEPTH IN THE HOLE, SIMULTANEOUSLY VARYING A VOLTAGE IN ACCORDANCE WITH VARIATIONS IN THE DIAMETER OF SAID BORE HOLE OPPOSITE SAID ZONE, SIMULTANEOUSLY RECORDING ON SAID STRIP IN CORRELATION WITH DEPTH IN THE HOLE A PLURALITY OF REFERENCE CURVES EACH CORRESPONDING TO A DIFFERENT VALUE OF SAID VARYING VOLTAGE AND THUS TO DIFFERENT VALUES OF THE HYDROGEN CONTENT IN THE MATERIAL SURROUNDING SAID SOURCE, IN ORDER TO PROVIDE A RECORD WITH THE INDUCED GAMMA RAY CURVE PLOTTED IN CORRELATION WITH CORRECTED REFERENCE CURVES. 